The Estrogen Endometriosis Connection

For more than 20 years, researchers thought it was just the ovaries that had the ability to produce steroid hormones in pregnancy. But a recent study on mice has found that after the embryo implants on the uterine wall, it is the uterus that manufactures the estrogen that is needed to maintain pregnancy.

Up until now, the uterus had never been thought of as an endocrine organ. So says Professor Indrani Bagchi who is affiliated with veterinary biosciences at the University of Illinois. Working alongside Bagchi was doctoral student Amrita Das. The study results were published in Proceedings of the National Academy of Sciences.

Local Source

According to Das, the estrogen that is synthesized by the uterus sustains the growth of the blood vessels found in this organ. After embryo implantation, this local source of estrogen works together with progesterone secreted by the ovaries to hasten the differentiation of the stromal cells of the uterus. This process is known as decidualization. The purpose of decidualization is to spur the growth of the blood vessels that are needed for the embryo's development.

Enough Estrogen

Bagchi and Das found that during the process of decidualization, the uterine stromal cells of mice gave off a larger amount of P450 aromatase; an enzyme that is crucial to the process of converting androgens into estrogen. Furthermore, the researchers discovered that in the case of pregnant mice that had their ovaries surgically removed, the uterus was still able to make enough estrogen to sustain the growth and decidualization of the blood vessels and tissues to support the continuation of the pregnancy.

Progesterone supplements are still needed to make sure these processes continue, however. This indicates that the local uterine estrogen is not enough in and of itself to sustain pregnancy.

Another finding suggests that the use of an aromatase inhibitor also stopped the process of decidualization. This is yet another proof that in order for a successful pregnancy outcome, there must be uterine estrogen production. According to Bagchi, this is an advantage since the right amount of estrogen is produced at the source independent of the need to rely on the ovaries for estrogen production.

If one had to depend on the ovaries for this purpose, the ovaries would need to put out a very high level of estrogen in order to make sure enough of the hormone were present in the uterus to encourage decidualization. But when there is a high systemic level of estrogen, the progesterone action is exacerbated.

Bagchi believes these findings may be of great significance for researchers studying endometriosis. In endometriosis, endometrial tissue of the type that is shed through the vagina during menstruation grows at body locations outside of the uterus, such as within the pelvic cavity and in the bowels where the result is the production of painful lesions at these sites.

The condition is provoked in part by very high levels of estrogen secreted by the endometrial tissues that grow at these sites external to the uterus. The uterus of a woman who doesn't have endometriosis will not produce estrogen except during pregnancy. When there are high levels of this hormone, progesterone activity is blocked and this can cause benign tissue growth such as is seen in endometriosis.